1. Field of the Invention
The present invention relates to duplexers used in, for example, mobile communication terminals, and more specifically, to a duplexer that includes a transmission filter with a ladder circuit configuration including a plurality of elastic wave resonators.
2. Description of the Related Art
To date, a duplexer that includes a surface acoustic wave filter has been widely used in, for example, mobile communication terminals. For example, Japanese Unexamined Patent Application Publication No. 2004-96250 described below discloses a surface acoustic wave filter having a ladder circuit configuration, as a transmission filter of a duplexer. In Japanese Unexamined Patent Application Publication No. 2004-96250, an antenna terminal, a transmission terminal, and a ground terminal are provided on a piezoelectric substrate. A transmission filter having a ladder circuit configuration is arranged so as to be connected between the transmission terminal and the antenna terminal and connected to the ground terminal. Here, a routing wiring line connected to the ground terminal is arranged between the ground terminal and the antenna terminal in such a manner as to be close to the antenna terminal. As a result, a coupling capacitor Cg is formed. It is stated that this coupling capacitor Cg allows attenuation in the vicinity of the high-frequency side of the pass band to be increased.
Attenuation in the vicinity of the high-frequency side of the pass band of a transmission filter can be increased by providing a coupling capacitor as disclosed in Japanese Unexamined Patent Application Publication No. 2004-96250.
On the other hand, a filter device in which a longitudinally coupled resonator-type elastic wave filter is connected to the antenna terminal through a one-port elastic wave resonator is widely used as the reception filter of a duplexer. If the coupling capacitor such as the one disclosed in Japanese Unexamined Patent Application Publication No. 2004-96250 is provided in a transmission filter, there is a problem in that isolation characteristics in the pass band of such a transmission filter are degraded. Further, there is also a problem in that attenuation outside of the pass band of a reception filter is not sufficiently increased.